The objectives of this research are (1) to further develop and extensively apply the thermal pulse decay (TDP) method to measure the blood perfusion in various organs and tissues; (2) to evaluate the influence of various vascular architectures on the convective heat transfer in the tissue; (3) to develop and evaluate a modified version of the TPPD method (MTPD) that is based on a more detailed description of heat transfer within the tissue that accounts for the convective contribution due to blood flow. The main measurement devices will be arrays of two or three thermistor probes in various configurations. Each microprobe in such arrays will be similar to the one used by the present TPD method. Thus the MTPD method will possess all of the advantages of the TPD method resulting from the small size of the probe and the transient nature of the measurement. The results obtained from the in vivo and in vitro (fixed tissue phatoms) experiments applying the MTPD method will be compared with other of blood flow measurement (microspheres, EM blood flowmeter, etc.) and the results critically discussed, analyzed, and evaluated, based on some new theoretical models of heat transfer in living tissues proposed recently by others and by the applicants. The proposed MTPD method of measuring local tissue perfusion has application in numerous areas of cardiovascular study. Quantitative blood flow measurements can be obtained in organs or tissues which have been proven difficult to evaluate in the past, e.g., cortico-medullary junction in the kidney, intestine, skin, etc. Of additional importance, the theoretical contribution of the proposed research will provide a better understanding of the modes of heat transfer in biological tissues.